The present invention relates to charged particle beam detection apparatus and in particular to a charged particle detector wherein a primary beam passes through the detector.
For general background information reference is made to: xe2x80x9cSem Notes #1xe2x80x9d (website: http://www.uga.edu/xcx9ccaur/semnotel.htm); and xe2x80x9cScanning Electron Microscopyxe2x80x9d, L. Reimer, Springer-Verlag, ISBN 3-540-13530-8 and ISBN 0-387-13530-8. For prior art particle detectors reference is made to: U.S. Pat. No. 4,149,074 Schliepe et al April 1979, U.S. Pat. No. 4,831,266 Frosien et al May 1989, U.S. Pat. No. 5,466,940 Litman et al November 1995.
A Scanning Electron Microscope (SEM) consists of inter alia, an electron source emitting a current of charged particles (the primary beam), an acceleration system for the charged particles, a steering and focusing system which brings the charged particles to a finely focused point on a sample and a scanning mechanism which causes the focal point to scan the sample in a controlled predefined manner. Charged particles which are created due to the interaction of the primary beam with the sample (secondary beam) scatter in the sample chamber. A detector collects these created charged particles (foremost secondary electrons or in short, SE""s) utilizing an electromagnetic field. It should be noted that a multitude of particles are created in the above-mentioned interaction, such as inter alia, backscattered electrons (BSE""s), Auger and X-Ray electrons, but are in following description not further discussed. In a widely common detection scheme, called in-lens detection, the secondary electrons (SE""s) are pulled back into the microscope, in where an electron detector is placed somewhere close to the original path of the primary beam inside the microscope (in-lens detector). Another detection scheme wherein the detector is positioned somewhere in the chamber is herein not further discussed by virtue of its substantially different operation.
There follows now a description of prior art in-lens detection schemes.
In FIG. 1 a prior art detector is shown, wherein scintillator 10 is positioned near or on light guide 11 and consists commonly of a phosphor or plastic or crystal scintillator. Light guide 11 is coupled to photomultiplier tube 12. Primary particle beam 13 passes through the light guide 11 and scintillator 10 by means of primary beam shielding 14. A protective sleeve 15 shields primary particle beam from the electrical field of the scintillator anode, biased at an electric potential of several kV""s above VCOLUMN. Column electric voltage potential (VCOLUMN) is the electric potential of the beam transport tube or structure surrounding the detector along the primary beam path. Both primary beam shielding 14 and protective sleeve 15 are at VCOLUMN electric potential. Thus, the primary beam does not experience any influence caused by electric fields originating from the significantly high electrical potential of the detector anode. After primary particle beam impinges on sample 16, held at a defined electric potential VSAMPLE, charged particles 17 are accelerated into lens 110, due to the electric potential difference between the sample and the column, and finally impinge on scintillator 10. The energy needed to create a signal from the charged particles on the scintillation material is gained, in prior art, by the above-mentioned electric potential difference between the column and the sample. FIG. 2 illustrates schematically the scintillator area in more detail. Scintillator 20 is attached to or forced on light guide 21. Photons generated by the scintillator are directed through light guide 21 toward photomultiplier tube 22 that generates the electric signal representative of the number of charged particles impinging on it. Primary beam shielding 23, through which the primary beam passes, is positioned inside a hole 24 in light guide 21. A multitude of charged particles 25 after being generated by the interaction between the primary particle beam and the sample are accelerated towards scintillator 20 by the electric potential difference between the column and the sample.
Hole 24 is substantially responsible for creating an area 26 in which photons generated by impinging charged particles 27 are obstructed from reaching photomultiplier tube 22. This area is therefore distinguished from the rest of the scintillator by its low efficiency properties, also commonly referred to as the xe2x80x9cshadowxe2x80x9d area of the primary beam shielding. Charged particles impinging onto the xe2x80x9cshadowxe2x80x9d area do contribute less to the generation of the image resulting from the transformation of photons to an electric signal by the photomultiplier tube, thus creating distinctive shadowing, detrimental to optimum detection performance.
There is accordingly a need in the art to provide for a system that substantially reduces or eliminates the shadowing effect of in-lens light guide scintillator detectors, and allow additional amplification of the signal without changing the general beam transport conditions, achieving improved and possibly uniform detection.
For purpose of clarity the acronyms SE and BSE are respectively used for secondary electron and backscattered electron in all discussions and claims below and more generally, relate to charged particles or electrons.
It should be noted that in the discussions below the phrase xe2x80x9cin-lens detectorxe2x80x9d is generally applicable to any detector wherein the primary beam passes through.
Furthermore, it is appreciated that the reference to Scanning Electron Microscope (SEM), does not limit or confine the present invention in any way and is generally applicable to any charged particle beam detection apparatus.
In accordance with the preferred embodiment of the present invention, there is provided a high efficiency, enhanced detecting light guide of an in-lens detector for SEM, having a variable electric potential on the scintillator surface and having an open area. In the preferred embodiment, instead of being set at VCOLUMN and open to the rest of the system, the scintillator surface is electrically isolated from its surroundings and biased at a higher electrical potential, bringing about impingement of electrons with greater energy. The higher energy results in more light being produced per electron and this leads to higher detection efficiency. The scintillator is shielded from the rest of the system by one, and preferably two grids. The external grid is set at VCOLUMN and thus prohibits disturbance of the primary beam. The second, inner grid may be set at a low electric potential, which will inhibit some, or all, of the electrons from reaching the scintillator. This grid may be used as a high pass energy filter for the incoming electrons.
The electric field created between the grid and the scintillator is used to direct the electrons away from the xe2x80x9cshadowxe2x80x9d region (26 of FIG. 2). This is accomplished by creating an open area instead of the xe2x80x9cshadowxe2x80x9d region. The open area results in shaping the light guide and scintillator into fork-like parts, with the two sides of the fork-like parts protruding as prongs. A primary beam shielding is situated in the open area between the prongs of the fork-like part facilitating passage of primary particle beam. The open area covers substantially the same area where the shadow of the primary beam shielding causes low efficiency detection of impinging electrons.
In accordance with another embodiment there is provided a conductive grid or mesh-sheet fitted in the open area between the prongs of the fork-like part. The electric potential, shape and position of the conductive grid can be determined to achieve one out of many desired possible interactions with charged particles (inter alia, SE and BSE electrons), such as e.g. deflection and generation of SE""s. Maintaining the conductive grid or mesh-sheet at a low enough electric potential below the kinetic energy of the fastest electrons originating from the sample induces deflection, away from the open area between the prongs of the fork-like part, consequently contributing toward substantially uniform electron detection. Thus achieving a substantial decrease of electrons not being detected, independent of where the electrons hit the detection area.
The conductive grid, situated in the open space, can optionally be replaced by a conductive metal foil, having for example, a tent-like shape, covering in various ways the open area of the fork-like part. Since, under some conditions, not all electrons with high kinetic energies will be deflected sufficiently by the electric potential of the foil to impinge on the high efficiency area of the detector, a given percentage will impinge on the foil. The foil, which may have a high emissive coating, will therefore produce SE""s by the impingement. These SE""s will be accelerated toward the scintillator anode biased at a positive electric potential, relative to the foil. Applying appropriate deflection means, being part of the lens optics, the SE""s but particularly BSE""s having high kinetic energies, can be deflected toward the conductive foil inducing impingement substantially only on the conductive foil so that no irreversible damage will be induced to the surface of the scintillator. Irreversible damage is potentially possible when electrons having high kinetic energy impinge on the scintillator in such manner, that the transformation process of electrons into photons will not constitute an ideal adiabatic process (a process that occurs without loss or gain of heat). Thermal damage due to non-adiabatic transformation of electrons into photons is also possible when the detected beam (the beam or stream of electrons emanating from the sample caused by interaction of primary particle beam impinging on the sample) exhibits a crossover point at or very near to the position of the scintillator. Secondary electrons produced on the foil are, in turn, attracted to the scintillator and spread over it to produce a signal from a large region. This embodiment is significant when the detected electron beam has a small geometrical cross-section (also called low-profile) on the detector. Spreading the electrons over the detector reduces local radiation damage on the scintillator. It also reduces artifacts in detection due to local variances in the scintillator.
The conductive foil thus effectively enables transformation of focused, high kinetic energy electrons into unfocused, low kinetic energy electrons. In addition, an increase in amount of generated electrons is achieved by a coating from a choice of suitable high emissive materials to produce more than one secondary electron for each electron impinging on it. Thus enhanced detection sensitivity is additionally achieved. In another embodiment the above-described functionality of the conductive foil is achieved by a known per se MCP (Micro Channel Plate) device, further increasing amount of generated SE""s. In addition, a spread of SE""s over substantially the entire scintillator area is achieved, thus avoiding non-linear scintillator response due to local saturation of the scintillator.
The primary particle beam is protected or shielded from the electrical field of the scintillator by a conductive sleeve or box, effectively encapsulating the scintillator. The protective box features a grid or mesh-sheet at the entrance side facing the sample to facilitate passing through of electrons emanating from the sample. Grid, protective box and primary beam shielding are electrically connected to an electric potential equal to the electric potential VCOLUMN in the vicinity of the detector. Inside the protective box is optionally accommodated one or more grids, biased at defined various electric potentials, lower than detected beam energy electric potential, to achieve variable high-pass energy filtering of electrons.
In another preferred embodiment, the grid closest to the scintillator is replaced by a Micro Channel Plate (MCP). The external face of the MCP is set commonly at VCOLUMN or VFILTER, depending if the system is used in a filtered (two grid) configuration or unfiltered (single grid) configuration. The back side of the MCP is set at the appropriate electric potential and the scintillator is set at a higher electric potential, typically, a few kV""s. Electrons striking the MCP, start a cascade which end with the emission of a substantial amount of electrons. These electrons are accelerated towards the scintillator anode of which the shadow region is open. In this configuration, the increase in efficiency is achieved by the multitude of electrons created by means of the MCP, rather than the energy of the electrons impinging onto the scintillator anode. Furthermore, the special xe2x80x9cunshadowedxe2x80x9d (open area) configuration of the light guide and scintillator resulting in a significantly homogeneous detection.
Thus, in accordance with one embodiment there is provided:
a high efficiency charged particle beam detector, comprising:
(a) a photon-detection element;
(b) a scintillator for producing photons from detected charged particles that impinge on said scintillator, the scintillator having an open area, substantially replacing a region in said scintillator manifesting low efficiency detection;
(c) a light guide associated with said scintillator for transferring said photons to said photon-detection element, said light guide having an open area, substantially conform in size and position with said open area of said scintillator
(d) a primary beam shielding facilitating passing through of said primary particle beam without substantial hindrance or affect from electrical field of said scintillator, said primary beam shielding accommodated near said open area; and
(e) a conductive part, accommodated within said open area of said light guide for inducing at least one out of various interactions between said detected charged particles and said conductive part.
Furthermore, in accordance with another embodiment, there is provided:
a high efficiency charged particle beam detector, for use with a Scanning Electron Microscope (SEM), comprising:
(a) a photon-detection element;
(b) a scintillator for producing photons from detected charged particles that impinge on said scintillator, the scintillator having an open area, substantially replacing a region in said scintillator manifesting low efficiency detection; comprising:
(i) an insulating base material;
(ii) a conductive layer disposed on said insulating base material, said conductive layer constituting a scintillator anode to bias said scintillator at a defined electric potential; and
(iii) a scintillating layer or element, disposed to or forced onto or under said conductive layer, or disposed between said conductive layer and said insulating base material;
(c) a light guide associated with said scintillator at one end of said light guide for transferring said photons to said photon-detection element at other end of said light guide, having an open area conform in size and position with said open area of said scintillator;
(d) said scintillator and said light guide having a hole through which said primary beam protrudes; and
(e) a protective sleeve encapsulating substantially said scintillator to shield said primary particle beam from the electrical field of said scintillator, comprising:
(i) a primary beam shielding having a hollow tube-like form, facilitating passing through of said primary particle beam without substantial hindrance or affect from electrical field of said scintillator, said primary beam shielding accommodated within said open area;
(ii) an electrically conducting box, having an open side from where said detected charged particles enter said box;
(iii) a protective grid or mesh-sheet covering said open side, electrically connected to said protective sleeve for achieving protection of immediate environment from electrical field of said scintillator anode; and
(iv) a filter grid or mesh-sheet assembly, accommodated in front of said scintillator, constituting more than one filter grid or mesh-sheet elements, each said filter grid or mesh-sheet elements electrically isolated and biased at a positive or negative electric potential for inducing various interactions between said detected charged particles and said filter assembly, wherein one or more elements of said filter grid or mesh-sheet elements is a MCP.
Furthermore, in accordance with yet another embodiment, there is provided:
a method for detecting charged particles, comprising:
(a) producing photons from detected charged particles that impinge on a scintillator, having an open area substantially replacing a region in said scintillator manifesting low efficiency detection;
(b) transferring said photons to a photon-detection element, having an open area, conform in size and position with said open area of said scintillator;
(c) facilitating passing through of the primary particle beam without substantial hindrance or affect from electrical field of said scintillator; and
(d) inducing at least one out of various interactions between said detected charged particles and said conductive part.
In above-mentioned method, alphabetic characters used to designate steps are provided for convenience only and do not imply any particular order of performing the steps.
As mentioned above, it should be noted that in above discussions the phrase xe2x80x9cin-lens detectorxe2x80x9d is generally applicable to any detector wherein the primary beam passes through or nearby and thus, the above referral to SEM does not limit or confine the present invention in any way. The invention is thus, in general, relevant for any charged particle beam detection apparatus, wherein the detection area embodies an area of low detection efficiency.